Glass ionomers used as dental restoratives have advantages over conventional resin containing composites and conventional resin reinforced glass ionomer restoratives for several reasons. For example, glass ionomers are tolerant to application on wet surfaces, have low shrinkage and are self-adhesive. For these reasons, they are said to, be easy and forgiving to use. Also, since glass ionomers contain polymers rather than monomers, there is no risk of acrylic monomers leaching out, which can lead to sensitization and allergic reactions. Furthermore, glass ionomers bond chemically to dental hard tissues, and may also provide a beneficial level of fluoride release, which helps to prevent recurrent caries.
A key weakness of commercial glass ionomers, however, is their relatively low biaxial flexural strength of only up to about 45 MPa, and low fracture toughness of only about 0.5 MPa·m1/2 which may lead to fracture at the edges of a restoration and, in the worst case, to bulk fracture of a restoration.
The inferior mechanical properties of conventional glass ionomers limit the range of application. Moreover, the inferior mechanical properties of conventional glass ionomers may lead to catastrophic failure when used as a dental restoration, such that the lifetime of the restoration may be significantly shortened.
A second problem with conventional glass ionomer hardening compositions is the stability of the polymer solution used. At the high concentrations needed to obtain sufficient strength, the polymer solutions tend to gel on storage and cannot then be mixed with a particulate glass powder.
A third problem is that the viscosity of the polymer solutions used in glass ionomer hardening compositions increases with increase in molecular weight, so that with high molecular weight polymers the polymer solution and particulate glass powder become hard to mix.
Yet a further problem is that many glasses presently available for use in glass ionomer formulations are too reactive towards acid and do not provide a long enough working time when ground to the required fine particle size.
U.S. Pat. No. 4,758,612 describes a dental cement containing a particulate aluminosilicate glass and a water soluble polymer having an average molecular weight from 1000 to 1,000,000, preferably from 10,000 to 25,000. Copolymers of acids such as acrylic acid, aconitic acid, itaconic acid, or citraconic acid are suggested for use in the invention.
It is the problem of the present invention to provide a hardening composition comprising a particulate glass and an acidic copolymer reactive with the particulate glass under aqueous conditions, wherein the acidic copolymer may either be                (i) dissolved in water to form an aqueous solution of the acidic copolymer in high concentration which has high stability during storage and may be easily mixed with a particulate glass powder or with an admixture of particulate glass and dried copolymer (a glass ionomer powder), or        (ii) dried and mixed with a particulate glass to provide a glass ionomer powderand wherein the glass ionomer hardening composition sets or hardens to form a hardened glass ionomer which provides improved mechanical properties over conventional glass ionomers, in particular increased biaxial flexural strength and fracture toughness, Klc.        
It is a further problem of the present invention to provide an acidic copolymer solution of the required high molecular weight and concentration which is stable over time with respect to gelling.
It is a further problem of the present invention to provide a use of a glass having an appropriate reaction speed when mixed with an acidic polymer solution, so as to allow suitable working and setting times when the glass is ground to a fine particle size.
As an additional desirable feature, hardening compositions which, when hardened, form a glass ionomer suitable for dental use, should be opaque to X-rays.